The present invention relates to antenna systems, and more particularly to a system in which a so-called center-feed television antenna is involved, and it is desired to couple a more or less standard or conventional rectangular waveguide to a coaxial transmission line.
In effectuating a coupling of a coaxial transmission line to a waveguide transmission line, the arrangement usually adopted is a so-called "T" transition of the standard rectangular waveguide to the coaxial line. In such a configuration, the inner conductor of the coaxial transmission line passes directly through the center of the rectangular waveguide with the coaxial "outers" shorted to both the top and bottom of the rectangular waveguide.
It will be understood by those skilled in the art that if a transmission line is terminated in an impedance different from its own characteristic impedance, reflection will occur and there will be standing waves of voltage and current along the line, which may be large if there is a considerable mismatch. This is the case in the rectangular-to-coaxial waveguide "T" junction since the characteristic impedance of the coaxial line is usually different from that of the rectangular waveguide.
It will further be understood by those skilled in the art that, in general, standing waves caused by mismatches are undesirable because they prevent maximum transfer of power. The solution to this difficulty is to obtain a match between the transmission lines, and for this purpose it is possible to use a properly located matching network.
However, as will also be appreciated, matching techniques are usually performed at a particular frequency. Thus, when the system is matched to that desired frequency, no reflections will occur at the frequency. However, how the system behaves at frequencies other than the desired design or matching frequency is also of significant concern. In many antenna applications, a device must have minimal reflections over a wide range of frequencies. A device with this important characteristic is sometimes referred to as "broad-band".
Accordingly, it is a primary object of the present invention to enable suitable coupling of different types of lines within an antenna structure or system and to insure that such device for coupling or matching purposes possesses a "broad-band" characteristic.
For purposes of the present discussion, the bandwidth may be defined as follows: Let fu and fl be the upper and lower frequencies of operation for which satisfactory performance is obtained. Let fc be the design frequency for which the device is matched. The bandwidth can then be represented by a percentage or ##EQU1##
For the earlier-noted television broadcast application, the coupling of a coaxial transmission line to a waveguide transmission line in the form of a so-called "T" junction had to have a bandwidth of ##EQU2## within a 1.1:1 VSWR (voltage standing wave ratio) circle. It should be especially noted that the 1.1:1 VSWR specification denotes the reflection characteristics of the device. Accordingly, the measured impedance should fall within the VSWR circle for the device to be considered to have appropriate or proper reflection characteristics.
It has sometimes been the practice in coupling a coaxial transmission line to a waveguide transmission line to provide a circular cylinder coupling or connecting device, such device having a larger diameter than the coaxial transmission line. This connecting device is referred to as a slug. However, such circular cylinder connection device or slug has not proven satisfactory in the waveguide-to-coaxial line junction context.
It is therefore another fundamental object of the present invention to achieve for an antenna coupling minimal reflections over a wide range of frequencies. More specifically, it becomes an object to realize the bandwidth noted above within a 1.1:1 VSWR circle.